XXXIIL— THE TRANSMITTED EFFECT OF PHOTIC 

 STIMULATION 



By 



Sir J. C. BosE, 



Assisted by 



Jyotiprakash Sircar, m.b. 



Plant organs exhibit, as we have already seen, a helio- 

 tropic curvature under direct stimulation. Still more inter- 

 esting is the transmitted effect of light giving rise to a cur- 

 vature. Thus if the tip of the seedling of wheat be exposed 

 to light, the excitation is transmitted lower down into 

 the region which acts as the responding organ. Growth 

 is very active in, this particular zone, and the change of 

 growth, induced by the transmitted effect of stimulus, brings 

 about a curvature by which the tip of the seedling bends 

 towards light. The seedling thus appears to be differentiat- 

 ed into three physiological zones subserving three different 

 functions. The tip is the perceptive zone, the inter- 

 vening distance between the tip and the growing region 

 is the zone of conduction, and the growing region is the 

 responsive zone. These differentiations are shown in a 

 striking manner by certain Paniceae, Setan'a for example. 

 In this seedling the tapering sheathing leaf or coty- 

 ledon is about 5 mm. in length, and it is the upper part 

 of the cotyledon that is most sensitive to light. Helow 

 the sheathing leaf is a narrow length which will 

 be distinguished as the hypocotyl, and where growth is 

 very active. The apex of the leaf perceives the stimulus, 



